The version is Grub2. However I had some of the details wrong in the OP. (Was writing the post from memory since I was away from the computer). The exact issue was that the computer boot looped immediately after the text "Grub Loading." was displayed on a line by itself. However, the problem does not occur without putting in the new drive for /dev/sdb. (This was wrong in the OP).

So, for clarity:

When removing /dev/sdb it boots perfectly fine.
When inserting the new /dev/sdb it boot loops after "Grub Loading."

I did eventually figure it out. For some reason, if I specified the boot device in the BIOS as /dev/sda then it booted OK. But if I left the BIOS to it's defaults (which put /dev/sdb at a higher preference) then it boot looped.

Still not sure why it happened though, because Grub was loading so the BIOS did find the correct drive eventually. Both /dev/sdb's had data on them and neither ever had anything grub related.